Radio astronomers help solve double-star conundrum

Sen—The Sun is unusual in being alone because stars commonly are found in families of two or more. Orbiting together around a common centre of gravity, they can have contrasting colours, offering attractive sights through a small telescope.

How these stars come to be tied up together has been a matter of some debate but now it has taken the power, not of an optical observatory, but of a radio telescope to help solve the conundrum.

Using the several dishes of the upgraded Karl G Jansky Very Large Array (VLA) in New Mexico, astronomers have detected previously-unseen binary companions of a pair of very young stars, known as protostars, that are just starting on the path of stellar evolution.

Around half of stars similar to the Sun are found in double or multiple-star systems, making it important to understand how they fit into the big picture of a galaxy’s development. The interactions between these stars, circling each other like pirouetting ice skaters, allows astronomers to discern much about their mass and make-up.

However scientists have been unable to say for sure which of competing explanations for the formation of double stars is the right answer.

John Tobin, of the U.S. National Radio Astronomy Observatory (NRAO), said: “The only way to resolve the debate is to observe very young stellar systems and catch them in the act of formation. That’s what we’ve done with the stars we observed, and we got valuable new clues from them,” he added.

These artist’s images show how a rotating disk of gas and dust, left, fragments to form a second protostar, centre, resulting in a binary star, right. Credit: Bill Saxton, NRAO/AUI/NSF

Tobin and an international team of astronomers found their answers by turning the VLA on a cosmic nursery of baby stars enshrouded in a nebula of gas and lying around 1,000 light-years from Earth. Their observations showed that two of the protostars had previously-undetected companions in the plane where the swirling disks that formed them would be expected to be. One of the systems also clearly had a disk surrounding both young stars.

The fresh evidence supports the idea that double-star systems form when a disk of gas and dust whirling around one young star fragments, forming another new star in orbit with the first. Young stars that still are gathering matter from their surroundings form disks like these, along with jet-like outflows that rapidly propel material in narrow beams perpendicular to the disk.

Tobin commented: “This fits the theoretical model of companions forming from fragmentation in the disk. This configuration would not be required by alternative explanations.”

The fresh results from the VLA are the latest evidence to give weight to the disk-fragmentation idea. In 2006, a separate team working with the VLA found an orbiting pair of young stars, each of which was surrounded by a disk of material. They noted that the two disks were aligned with each other along the same plane.

The Very Large Array in New Mexico that made the discovery. Credit: NRAO/AUI

Last year, Tobin and his colleagues found a large circumstellar disk forming around a protostar in the initial phases of star formation. This showed that disks are present early in the process of star formation, something that is necessary to allow binary pairs to form through disk fragmentation.

Team member Leslie Looney of NRAO and the University of Illinois said: “Our new findings, combined with the earlier data, make disk fragmentation the strongest explanation for how close multiple star systems are formed.”

The new research was made possible by a major upgrade of the VLA, which is named after Jansky, the American physicist who first noticed in 1931 that radio waves were being broadcast from the heart of the the Milky Way.

Observations made at the observatory’s highest-frequency band, from 40 to 50 GHz, allowed the scientists to tune in to signals emitted by dust in the disks surrounding the young stars.

Claire Chandler of NRAO said: “The increased sensitivity of the VLA, produced by a decade-long upgrade project completed in 2012, made the new discovery possible.” The findings of the team, made up of astronomers from the U.S., Mexico and the Netherlands, are published in the Astrophysical Journal.